The measurable volume of a magnetic resonance imaging recording is limited in all three spatial directions in a magnetic resonance scanner as a result of physical and technical conditions such as e.g. a limited magnetic-field homogeneity and a nonlinearity in the gradient field. Therefore a recording volume—a so-called field of view (FoV)—is restricted to a volume in which the aforementioned physical features lie within a predetermined tolerance range and hence afford the possibility of using conventional measurement sequences for faithful imaging of the object to be examined. However, the field of view restricted thereby is significantly shorter than that volume delimited by the ringed tunnel of the magnetic resonance scanner, particularly in the x- and y-directions, i.e. perpendicularly to a longitudinal axis of a tunnel of the magnetic resonance scanner. In the case of conventional magnetic resonance scanners, a diameter of the ringed tunnel is e.g. approximately 60 cm, whereas the diameter of the conventionally used field of view, within which the aforementioned physical features lie within the tolerance range, is approximately 50 cm.
In many applications of magnetic resonance scanners this inadequacy—the fact that there cannot be a faithful image of the measurement object in the edge region of the tunnel of the magnetic resonance scanner—does not constitute a major problem because, if there is only a magnetic resonance recording, the region of the object to be examined can usually be arranged in the magnetic resonance scanner such that this region is not situated at the edge of the tunnel but rather in the center of the tunnel where possible, in the so-called isocenter of the magnetic resonance scanner. However, in the case of hybrid systems, such as e.g. a hybrid system consisting of a magnetic resonance imaging scanner and a positron emission tomography scanner—a so-called MR/PET hybrid system—it is often of the utmost importance to determine structures of the examination object as precisely as possible, even in the edge region.
By way of example, in the case of a MR/PET hybrid system, the human attenuation correction is of decisive importance. The human attenuation correction is used to establish the intensity attenuation of the photons, which are emitted after an interaction between positrons and electrons, on their path through absorbing tissue to the detector and the received signal from the PET is corrected by precisely this attenuation. To this end a magnetic resonance recording is acquired, which images the complete anatomy of the object to be examined in the direction of the high-energy photons emitted by the positron emission tomography.
Thus, the anatomy of the object to be examined should also be ascertained as precisely as possible in the edge region of the tunnel of the hybrid system. Structures that are situated in these regions are for example mainly the arms in the case of patients to be examined, which arms may be arranged in the edge region in the vicinity of the tunnel inner wall in the hybrid system.
In the patent application with the application number DE 10 2010 006 431.9, from the same inventor and the entire contents of which are hereby incorporated herein by reference, a method is provided for determining a position of a portion of an examination object in a magnetic resonance scanner. The portion of the examination object is arranged at the edge of the field of view of the magnetic resonance scanner. In the method, at least one slice position for a magnetic resonance image is determined automatically, in which slice position the B0 field at the edge of the magnetic resonance image satisfies a predetermined homogeneity criterion. Furthermore, a magnetic resonance image is recorded in the specific slice position, which contains the portion at the edge of the field of view. The position of the portion of the examination object is determined automatically by the position of the portion in the recorded magnetic resonance image.
Furthermore, the prior art has proposed a method by Delso et al. for compensating the missing information in the magnetic resonance image, which information is missing as a result of the limited field of view, by segmenting the body contours using uncorrected PET data (G. Delso et al., Impact of limited MR field-of-view in simultaneous PET/MR acquisition, J. Nucl. Med. Meeting Abstracts, 2008; 49, 162P).
Since the field of view in a magnetic resonance scanner is limited to a volume in which the magnetic field inhomogeneity and the nonlinearity of the gradient field lie within specified ranges, the prior art has presented different correction algorithms for extending the field of view. By way of example, a gradient distortion correction is proposed in Langlois S. et al., MRI Geometric Distortion: a simple approach to correcting the effects of non-linear gradient fields, J. Magn. Reson. Imaging 1999, 9(6), 821-31 and in Doran S J et al., A complete distortion correction for MR images: I. Gradient warp correction, Phys. Med. Biol. 2005 Apr. 7, 50(7), 1343-61. Furthermore, a corresponding B0-field correction is proposed in Reinsberg S A, et al., A complete distortion correction for MR images: II. Rectification of static-field inhomogeneities by similarity-based profile mapping, Phys. Med. Biol. 2005 Jun. 7 50(11), 2651-61.